This invention relates to high temperature, oxidation-resistant, aluminum containing oxide-dispersion-strengthened (ODS) alloy-supported catalyst compositions for oxidative dehydrogenation processes and a method of using such catalysts in the presence of hydrocarbons. More particularly, this invention relates to compositions of MCrAlY-supported catalysts for the production of olefins by oxidative dehydrogenation of hydrocarbons in short-contact time reactors (SCTRs).
Dehydrogenation of hydrocarbons is an important commercial process. Dehydrogenation is the process used to convert aliphatics to olefins, mono-olefins to di-olefins, cycloalkanes to aromatics, alcohols to aldehydes and ketones, aliphatics and olefins to oxygenates, etc., by chemically removing hydrogen from the starting molecule(s). In more practical terms, this process has been used to produce commercially many of the precursors of products such as detergents, gasolines, pharmaceuticals, plastics, polymers, synthetic rubbers and many others. For example, polyethylene is made from ethylene, which is made from the dehydrogenation of ethane (i.e. aliphatic to olefin). More ethylene is produced in the U.S. than any other organic chemical. Thus, it is easy to appreciate the significance of the dehydrogenation process to industry.
Traditionally, the dehydrogenation of hydrocarbons has been carried out using steam cracking or non-oxidative dehydrogenation processes. Thermal or steam cracking is an extremely energy intensive process that requires temperatures in excess of 800xc2x0 C. About 1.4xc3x971015 BTU""s (equivalent to burning 1.6 trillion ft3 of natural gas) are consumed annually to produce ethylene. In addition, much of the reactant (ethane) is lost as coke deposition. Non-oxidative dehydrogenation is dehydrogenation whereby no molecular oxygen is added.
Oxidative dehydrogenation of hydrocarbons (ODH) with short contact time reactors is an alternative to traditional steam cracking and non-oxidative dehydrogenation processes. During an ODH reaction, an oxidant is co-fed with saturated hydrocarbons. Typically the oxidant is a gas containing oxygen. The oxygen-containing gas may be pure molecular oxygen, air, oxygen-enriched air, oxygen mixed with a diluent, and so forth. However the presence of a diluent such as an inert gas in the oxidant increases the reactor and equipment size. The oxidant in the desired amount may be added in the feed to the dehydrogenation zone and the oxidant may also be added in increments to the dehydrogenation zone. Gas hourly space velocity (GHSV) is typically from 20,000 to 10,000,000 hrxe2x88x921. For the present process, GHSV is defined by the ratio of the volumetric flow rate (m3/hr) of gaseous feed at normal pressure and temperature over the catalyst bed volume (m3). The contact time of the reactants with the catalyst is typically in the 10-200 ms range. The reaction pressure is typically between 1 and 50 bars.
The capital costs for olefin production via ODH are significantly less than with the traditional processes, because ODH uses simple fixed bed reactor designs and high volume throughput. In addition, ODH is an autothermal process, which requires no or very little energy to sustain the reaction. Energy savings over traditional, endothermal processes can be significant if the heat produced with ODH is recaptured and recycled. Often, the trade-off for saving money in commercial processes is loss of yield or selectivity; however, the ODH reactions are comparable to steam cracking in olefin selectivity and alkane conversion.
As mentioned above, ODH is an exothermic reaction, and temperatures at typical reaction conditions in excess of 1,000xc2x0 C. may be required for successful operation. It is known that ceramic monolith catalyst supports are susceptible to thermal shock; that is, either rapid changes in temperature with time or substantial thermal gradients across the catalyst structure. Catalysts and catalyst supports for use in such a process must therefore be very robust, and avoid structural and chemical breakdown under the relatively extreme conditions prevailing in the reaction zone.
U.S. Pat. No. 5,639,401 discloses a porous monolithic foam catalyst support of relatively high tortuosity and porosity, preferably comprising at least 90 wt % zirconia for thermal shock resistance.
Complete oxidation of hydrocarbons, such as occur in automobile catalytic converters, also require catalysts, which function at high space velocities and also are stable at elevated temperatures of greater than about 700xc2x0 C. U.S. Pat. No. 5,511,972 discloses a catalyst structure that is effective under the severe conditions encountered in automobile catalytic converters. The catalyst structure comprises a ferrous alloy as the catalyst support. The ferrous alloy contains aluminum, which forms micro-crystals or whiskers of alpha-alumina on the alloy surface when heated in air. A washcoat of gamma-alumina is added to the alpha-alumina surface followed by the deposition of palladium.
Materials such as oxide-dispersion-strengthened (ODS) alloys can withstand high temperatures ( greater than 700xc2x0 C.) similar to those used in the ODH reaction system. As an example of ODS alloys, MCrAlY alloys have been used as a thermal coating or thermal barrier in high-temperature or corrosive environments such as diesel exhaust systems or gas turbine engines. As disclosed by Czech, et al., in Surface and Coatings Technology, 108-109 (1998) p. 36-42, stationary gas turbine engines for electric power generation operate at gas inlet temperatures that are as high as those in the ODH reaction zone. The turbine blades are subjected to very high thermal and mechanical loads and are additionally attacked by oxidation. To deal with the mechanical loads, the base material of the turbine blades is metallic in composition. To deal with the thermal and chemical stresses, the turbine blades have a coating with an MCrAlY composition, where M comprises Ni and/or Co, as a protective overlay coating against oxidation. Additional coatings may be added as thermal barriers. The overlay coatings are typically applied by either Low Pressure Plasma Spray or Vacuum Plasma Spray. The base material is protected in operation by an alumina scale, which forms from the overlay coating.
A MCrAlY alloy is comprised of chromium, alumina, yttrium and another metal or metal alloy M, with the metal preferably selected from the group of Ni, Co, Fe. The aluminum in MCrAlY forms the oxide scale. As a major constituent of the alloy, it provides a reservoir from which the alumina scale is constantly replenished. Replenishment, or film growth, is controlled by oxygen diffusing inwardly along alumina grain boundaries. The oxidation rate of MCrAlY is directly proportional to the formation rate of the alumina scale on its surface. Scale formation is attributed to the aluminum""s activity and its diffusivity in the alloy. This activity is increased by the presence of chromium, which also enhances diffusion rate of the metal M. Adding chromium lowers the amount of aluminum needed to form and maintain the protective oxide film. If the aluminum content were increased instead of adding chromium, the MCrAlY alloy would show signs of brittleness.
FeCrAlY alloys exhibit high corrosion resistance and high sulfadation resistance. For optimum protection from hot corrosion, CoCrAlY alloys are preferred. xe2x80x9cHot corrosionxe2x80x9d or high temperature corrosion is a form of corrosion that does not require the presence of a liquid electrolyte. An example of hot corrosion is oxidation. Nonetheless, CoCrAlYs are preferably limited to applications operating at temperatures below 927xc2x0 C. NiCrAlYs can be used in a slightly higher temperature range than CoCrAlYs (up to 982xc2x0 C.) and offer better oxidation protection than CoCrAlYs. The shortfalls of these two alloys can be overcome by substituting a small percentage of one for the other, or making either a NiCoCrAlY or CoNiCrAlY alloy.
In addition to combining M-base alloys, other metals known to improve the alloys"" characteristics (i.e. silicon, hafnium, tantalum, and platinum) may be added. For example, silicon is a temperature suppressant and in small amounts is known to promote alumina scale adherence and, in some cases, form an oxide film of its own. Hafnium behaves in a manner similar to yttrium. After oxidizing to form hafnia needles, it locks the alumina scale in place. In some alloy systems, hafnium may be freely substituted for yttrium. Tantalum may be added to an MCrAlY alloy to improve the alloys"" high-temperature capabilities and resistance to sulfidation and hot corrosion. Platinum may be added to an MCrAlY alloy to increase its oxidation and hot-corrosion properties for operating at temperatures up to 1093xc2x0 C.
In the methods that employ catalysts for oxidative dehydrogenation of hydrocarbons to olefins, catalytic metals are typically dispersed throughout a ceramic oxide support. Ceramic oxides however, are known to have relatively low thermal conductivities. This poses a problem because of hot spots, in which the temperature is higher in localized regions than in the remaining part of the catalyst bed. Hot spots can form if catalyst loading or metal dispersion or resulting activity is uneven. These hot spots can give rise to secondary reactions, such as the total combustion of the starting material, or lead to the formation of undesired by-products, which can be separated from the reaction product only with great difficulty, if at all. Hot spot formation prevents attaining the desired process conditions, as well as leads to undesired reactions resulting in poor product yields, and in the worst case, in uncontrolled regimes i.e., runaway reactions and abrupt shutdowns.
A low gas flowrate is undesirable in especially short contact time reactors, because their processes are volume dependent (i.e. they operate at high gas space velocity). Accordingly, there is a continuing need for better, more economical processes and catalysts with good thermal and mechanical stability for the oxidative dehydrogenation of hydrocarbons, in which the catalyst retains a high level of activity and selectivity to olefins under conditions of high gas space velocity and elevated pressure.
In order to operate at very high flow rates, high pressure and using short contact time reactors, catalysts should be highly active, have excellent mechanical strength, resistance to rapid temperature fluctuations, thermal shocks and thermal stability at oxidative dehydrogenation reaction conditions.
The catalysts and methods of the present invention overcome some of the drawbacks of existing catalysts and processes for converting light hydrocarbons to olefins. The present ODS alloy-supported catalysts are expected to demonstrate greater thermal stability than ceramic oxide-supported catalysts and give comparable olefin yield to conventional oxidative dehydrogenation catalysts under conditions of high gas space velocity and elevated pressure. Another advantage provided by the preferred new catalysts and processes is that they are economically feasible for use under commercial-scale conditions.
The present invention provides a catalyst system for use in ODH that allows high conversion of the hydrocarbon feedstock at high gas hourly space velocities, while maintaining high selectivity of the process to the desired products. For the purposes of this disclosure, all listed metals are identified using the CAS naming convention.
In accordance with a preferred embodiment of the present invention, a catalyst for use in ODH processes includes a MCrAlY support. M is preferably a base metal, or combination of base metals. A base metal is herein defined as a non-Group VIII metal, with the exception of iron, cobalt and nickel. Suitable base metals include Group IB-VIIB metals, Group IIIA-VA metals, lanthanide metals, iron, cobalt and nickel. M preferably is iron, cobalt, nickel, indium, or manganese, and still more preferably iron, cobalt, or nickel. Additionally, the catalyst may optionally include a promoter such as a Group VIII metal. Suitable Group VIII promoters include Ru, Rh, Pd, Os, Ir, and Pt. The promoter is preferably deposited on the MCrAlY support.
Preferably, a millisecond contact time reactor, such as are known and described in the art, is used. By way of example only, operation of a millisecond contact time reactor is disclosed in detail in co-owned and co-pending U.S. Pat. No. 6,461,539 B1, filed Oct. 16, 2000 and entitled xe2x80x9cMetal Carbide Catalysts and Process for Producing Synthesis Gas,xe2x80x9d which is incorporated herein by reference in its entirety.
In accordance with another preferred embodiment of the present invention, a method for the production of olefins includes contacting a preheated alkane and oxygen stream with an MCrAlY-supported catalyst, under conditions sufficient to initiate the oxidative dehydrogenation of the alkane (the preheat temperature preferably being between 25xc2x0 C. and 800xc2x0 C.), maintaining a contact time of the alkane with the catalyst for less than 200 milliseconds, and maintaining an oxidative dehydrogenation-favorable alkane:oxygen molar ratio.
These and other embodiments, features and advantages of the present invention will become apparent with reference to the following description.